In fretted stringed musical instruments, one or more strings are stretched under tension across the main body of the instrument, which amplifies the sound of the vibrating string. One end of the string is anchored at one side of the main body (often referred to as the ‘bridge’), and the other end is anchored at the end of the neck furthest from the main body (often referred to as the ‘nut’). A series of ridges (or ‘frets’) running transverse to the neck (the ‘fretboard’) are positioned at various intervals, underneath the strings. The strings do not touch the frets, even during vibration.
The pitch of a plucked string is determined by the relationship of the tension of the string, its mass per unit length (which is a function of the string's diameter), and the length of the string available for vibration (effective length). This length is the distance between the bridge and the nut.
On fretted stringed musical instruments, one of the player's hands is used for plucking or striking the strings while the other one may press upon one or more strings of the instrument with one or more fingers to cause the string to engage the frets along the neck of the guitar or other musical instrument. This procedure reduces the effective string length to the distance between the bridge and the fret, allowing the musician to change the pitch as called for by the musical score.
Stringed instruments are normally played in a standard tuning; when no strings are depressed upon frets (termed ‘open’ strings, played without fingering), they are under defined tension to produce a standard series of pitches. A device called a ‘capo’ (or ‘capotasto’) is often used to mechanically depress the strings onto a fret, effectively shortening the length of the strings. Most capos depress all strings, though there is occasionally a need for only a selection of stings to be depressed. This makes a greater variety of sounds available to the musician.
There are a great number of patents issued for innovative capo designs, though fewer for capos that allow only a selection of strings to be depressed (e.g. U.S. Pat. No. 5,623,110, NL8900025). One major drawback of such designs is that although they allow a selection of strings to be depressed, the capo is only fitted across one fret, and so it is impossible to depress different strings at different frets without using multiple capos, which would impede playing. One capo design allows different strings to be depressed at different frets (GB2349010), but this would also impede the fretting hand as it moves across the device.
Further innovations have involved the use of a multitude of individual capo elements that depress only one string each, and can be positioned anywhere along the neck of the instrument (U.S. Pat. No. 6,998,526, CA2244647, CA2213187, U.S. Pat. No. 6,013,868, GB2393315, U.S. Pat. No. 5,056,397). These designs all focus on the mechanical means of depressing the string onto the fret, and all require the modification of the instrument itself, by requiring magnets within the instrument neck (U.S. Pat. No. 6,998,526, CA2244647, CA2213187, U.S. Pat. No. 6,013,868), mechanical fixings (GB2393315), or specially profiled frets (U.S. Pat. No. 5,056,397).
There remains a need for a means of mechanically altering the pitch of selected individual strings, at different positions along the instrument neck, which does not impede the fingering hand movement along the neck, and does not require any modification of the instrument itself.